Fabric Shortest Path First (FSPF) protocol in Fibre Channel networks is a link state path selection protocol that uses link cost as a metric while calculating routes through a network. Link cost in FSPF is calculated using the bandwidth of the link between the switches. FSPF keeps track of the state of the links between switches in the fabric. The protocol computes the total cost of multiple paths from a switch to the other switches in the fabric by adding the cost of the individual links traversed by the path and choosing the path with the lowest cost. FSPF networks are often built to have multiple data paths between switches in a network of switches. This increases the available bandwidth to destination while providing redundancy to deal with failures.
When networks are built with multiple data paths, they may have multiple equal cost multi-paths (ECMPs) between an origin and a destination. ECMPs describe a routing situation where packet forwarding to a single destination from a single origin can occur over multiple “best paths.” In ECMPs, the best paths have the lowest cost, where cost is determined by bandwidth. As the bandwidth of a link increases, the cost decreases. However, when only bandwidth is used to calculate the cost of a link, it is possible to have multiple paths with identical costs even though there might be different numbers of switches along each individual ECMP. It is not always desirable to consider paths with different numbers of switches as ECMPs even though they have the same lowest cost as determined by bandwidth alone. This is even more poignant when some of the links are long distance WAN links.